1. Technical Field
The present disclosure relates generally to radio frequency (RF) circuitry, and more particularly, to modified current mirror circuits for reduction of switch time.
2. Related Art
Wireless communications systems are utilized in a variety contexts involving information transfer over long and short distances alike, and a wide range of modalities for addressing the particular needs of each being known in the art. As a general matter, wireless communications involve an RF carrier signal that is variously modulated to represent information/data, and the encoding, modulation, transmission, reception, de-modulation, and decoding of the signal conform to a set of standards for coordination of the same.
Fundamental to any wireless communications systems is a transceiver, that is, a combined transmitter and receiver circuitry. More particularly, in a digital data communications system, the digital baseband system of the transceiver encodes the digital data to an analog baseband signal, and modulates the baseband signal with an RF carrier signal. Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the digital data represented by the baseband signal. A transmitting antenna connected to the transmitting transceiver converts the electrical signal to electromagnetic waves, while a receiving antenna connected to the receiving transceiver converts the electromagnetic waves to an electrical signal. In most cases, the transceiver circuitry itself does not generate sufficient power or have sufficient sensitivity necessary for communications. Thus, additional circuits are referred to as a front end is utilized between the transceiver and the antenna. The front end includes a power amplifier for boosting transmission power, and/or a low noise amplifier to increase reception sensitivity.
The RF amplifier, particularly those utilizing metal oxide semiconductors (MOS), may incorporate a current mirror circuit to set the bias point of the RF amplifier transistor. The current mirror circuit typically includes a pair of transistors coupled together such that the current through one of the devices matches, or mirrors the current in the other device. The mirror transistor is connected to the gate of the RF amplifier transistor over a mirror resistor, while the mirror transistor is connected to a control circuit that turns on and turns off the mirror transistor within a specific timeframe.
There is understood to be a residual capacitance comprised of a combination of the mirror transistor gate capacitance and the coupling capacitor that carries the RF signal to the RF amplifier transistor gate. Such residual capacitance, together with the aforementioned mirror resistor, defines an RC time constant, which significantly slows the transient response of the current mirror/biasing circuit. Although a reduction of the mirror resistor may reduce the RC time constant and hence the delay in the transient response, the mirror resistor must also be sufficiently high to avoid degradation in signal quality at the gate of the RF amplifier transistor.
More particularly, a low resistance value for the mirror resistor negatively affects performance parameters such as noise figure, while a high resistance value for the mirror resistor results in an extended transition period between the on and off states in the RF amplifier transistor. In some prior implementations, a switch may short the RF amplifier transistor gate voltage to ground when turning off the transistor, but the transient response when turning on the transistor would not be improved.
Thus, as a general matter, optimizing for a short transient response and improving the noise figure, particularly with respect to the mirror transistor, are mutually exclusive. Accordingly, there is a need in the art for independent optimization of the transient response and the noise figure of the RF amplifier circuit with the bias point set by the current mirror circuit.